Outstanding Ferroelectricity in Sol-Gel-Derived Polycrystalline BiFeO₃ Films within a Wide Thickness Range

As a promising lead-free ferroelectric, BiFeO₃ has a very large intrinsic polarization of ∼100 μC/cm², enabling its great potential in electronic applications especially in a film format. In this sense, reliable ferroelectric properties are desired; however, pure-phase BiFeO₃ films are notorious for their large leakage current, especially of those processed by using the sol-gel method─a facile and industrially scalable method for film preparation. In this study, a protection layer, which can be easily integrated in the sol-gel process, is used to ensure the acquirement of remnant polarization of ∼65 μC/cm² in ∼200 nm BiFeO₃ thin films, whereas O₂ annealing can enhance that to ∼120 μC/cm² in ∼400-700 nm films. Reliable ferroelectricity of BiFeO₃ films on Si wafers within a wide thickness range was thus achieved. The obtained ferroelectricity is among the best-achieved properties to date of BiFeO₃ films for both thin and intermediate thicknesses, including both chemically and physically derived. These results are helpful to advance potential use of sol-gel-processed BiFeO₃ films in electromechanical devices with different desired thicknesses.